Transistors used for most flat panel displays typified by a liquid crystal display device and a light-emitting display device are formed using silicon semiconductors such as amorphous silicon, single crystal silicon, and polycrystalline silicon provided over glass substrates. Further, transistors formed using the silicon semiconductors are used in integrated circuits (ICs) and the like.
In recent years, a technique in which, instead of a silicon semiconductor, a metal oxide exhibiting semiconductor characteristics is used for transistors has attracted attention. Note that in this specification, a metal oxide exhibiting semiconductor characteristics is referred to as an oxide semiconductor.
For example, a technique is disclosed in which a transistor is manufactured using zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductor and the transistor is used as a switching element or the like of a pixel of a display device (see Patent Documents 1 and 2).
Meanwhile, it has been pointed out that hydrogen is a source for supplying carriers particularly in an oxide semiconductor. Therefore, some measures need to be taken to prevent hydrogen from entering the oxide semiconductor at the time of forming the oxide semiconductor. Further, variation in a threshold voltage is suppressed by reducing the amount of hydrogen contained in the oxide semiconductor film or a gate insulating film in contact with the oxide semiconductor (see Patent Document 3).
Further, water is a source for supplying hydrogen. Therefore, by providing a silicon nitride film having a property of blocking water over a transistor including an oxide semiconductor film, entry of water from the outside to the oxide semiconductor film can be prevented.